Greetings.
Regarding your question about other alien species costumes it would depend on what you want to do with them. For the 501st, I think that they allow Weequay skiff guards (though don't quote me on that) but I don't think they'd allow a Pau'an of any kind other than, possibly, Tion Medon. But, again, I can't say that for certain. If you're not wanting to do a skiff guard or Tion Medon and want to do a more generic Weequay/Pau'an/whatever you could do so in the Rebel Legion with a pilot or Jedi (or, I would imagine, a Rebel trooper like Endor, Hoth or Fleet). You could also do a Sora Bulq costume for the Rebel Legion-- Sora Bulq was a Weequay Jedi.